Save the files 2up.sty|tex in a directory, which is part of your
local TeX tree.
Then do not forget to run texhash to update this tree.
For more information see the documentation of your LATEX distribution
on installing packages into your LATEX distribution or the
TeX Frequently Asked Questions:
(http://www.tex.ac.uk/cgi-bin/texfaq2html?label=instpackages).